X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcGadt.lhs;h=48b2b0623cd6455cba11c982be5e06cdc096cdfe;hb=72d043c04c1b0db0896e2c876a1544434f0428ec;hp=b556e89e3273ed15e45c2ad32e4d8479f7979933;hpb=1370a4a360ec154b390c1da21d40e446739b24c8;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcGadt.lhs b/compiler/typecheck/TcGadt.lhs
index b556e89..48b2b06 100644
--- a/compiler/typecheck/TcGadt.lhs
+++ b/compiler/typecheck/TcGadt.lhs
@@ -237,11 +237,15 @@ fixTvCoEnv in_scope env
       -- then use transitivity with the original coercion
 
 -----------------------------
+-- XXX Can we do this more nicely, by exploiting laziness?
+-- Or avoid needing it in the first place?
 fixTvSubstEnv :: InScopeSet -> TvSubstEnv -> TvSubstEnv
-fixTvSubstEnv in_scope env
-  = fixpt 
+fixTvSubstEnv in_scope env = f env
   where
-    fixpt = mapVarEnv (substTy (mkTvSubst in_scope fixpt)) env
+    f e = let e' = mapUFM (substTy (mkTvSubst in_scope e)) e
+          in if and $ eltsUFM $ intersectUFM_C tcEqType e e'
+             then e
+             else f e'
 
 ----------------------------
 tryToBind :: TyVarSet -> TyVar -> BindFlag